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easyBasicsSometimes asked

Count Uppercase Letters

Basics · 6 min

Given a string s, return how many uppercase letters (A–Z) it contains.

In plain English

One pass; bump a counter for each character in the 'A'..'Z' range.

Examples

Example 1

Input: s = "Hello World"
Output: 2
Note: H and W.

Example 2

Input: s = "abc"
Output: 0
Note: None.

Example 3

Input: s = "ABC"
Output: 3
Note: All upper.

Constraints

0 ≤ s.length ≤ 10^4

Clarifying questions1First question: ASCII letters only?Explore questions

Q1.ASCII letters only?
Yes; Unicode uppercase would use isUpperCase.

Quick interview revision

Understand

30-second explanation

Build the interview-ready mental model before comparing implementations.

Scan and count characters where `ch >= 'A' && ch <= 'Z'`.

Interview contextWhat is testedWhere candidates slipHow to stand outExplore interview context

What's being tested

Whether you write a clean range-membership tally.

Where candidates slip

Range slip including lowercase or symbols.

How to stand out

Note it generalises to count-upper-lower with a second counter.

Choose

Compare the available approaches

Use constraints and trade-offs to select the route you can defend in an interview.

Learn & trace

Build each solution step by step

Move from the core idea to a visual trace, implementation, proof, and edge cases.

03.1

Approach 1 of 1 · Optimal

Range count

TimeO(n)SpaceO(1)

This is the standard, expected solution — lead with it.

Stuck? Progressive hintsReveal one at a time

1Hint 1Explore hint

What ASCII range holds the capital letters?

2Hint 2Explore hint

One counter, bump on a range match.

3Hint 3Explore hint

Ignore lowercase, digits, and symbols.

A · Core idea

Count characters in the uppercase ASCII range 'A'..'Z' with a single counter — the simplest range-membership tally.

Why a range check. ch >= 'A' && ch <= 'Z' is an O(1) test that includes only the 26 capital letters; lowercase letters, digits, spaces, and symbols all fail it and are ignored.

Why one pass, no early exit. A total count requires visiting every character. There's no shortcut without preprocessing.

It's the count-upper-lower problem reduced to just the uppercase bucket.

B · Visual dry run

Interactive dry run

Select a frame to connect the decision with its visual state.

s = "Hi World"

A-Z

Statecount=1

Final result2

C · Code with teaching comments

solution.javaJava

Range count — optimal

1
public int countUpper(String s) {
2
    int c = 0;
3
    for (char ch : s.toCharArray()) {
4
        if (ch >= 'A' && ch <= 'Z') c++;
5
    }
6
    return c;
7
}

D · Why it works and complexityO(n) timeO(1) spaceOpen the invariant and proofExplore the proof

ComplexityO(n) time, O(1) space — one range comparison per character.

E · Edge cases and pitfalls5s = "abc" — None → 0.Explore edge cases

Edge cases handled

s = "abc"None → 0.
s = "ABC"All upper → 3.
s = ""Empty → 0.

Pitfalls

Including lowercase letters by mistake.
Counting non-letters.

Practice

Try the solution yourself

Edit the optimal implementation, test another input, and observe the result.

Editable code previewpublic int countUpper(String s) { int c = 0; for (char ch : s.toCharArray()) { if (ch >= 'A' && ch <= 'Z') c++; }Explore the playground

Try it — Java· run a complete program; method snippets need a driver

Avoid mistakes

Recognize the interview traps

Understand why common attempts fail before reviewing the corrected implementation.

Including lowercase letters by mistake.
Counting non-letters.

Revise

Carry the pattern into the interview

Quick interview revision

Pattern to remember

ASCII-range count

Code pattern to memorize

c=0; for ch: if 'A'<=ch<='Z': c++

Rules to remember

1Uppercase: A-Z.
2One counter.
3Skip everything else.

Use this pattern when

Range-membership counting

Interview takeaway

Range-membership counting is one comparison per character.

Continue

Transfer the pattern to another problem

Use a direct variation or pattern sibling to prove the idea is reusable.

Transferable patternA core fresher pattern: ASCII-range count. Master it on small inputs and it transfers to harder problems.

Practice queueDirect variations and pattern siblings

Direct follow-ups

Count Upper and Lower
Add the lowercase counter.

easyBasicsSometimes asked

Count Uppercase Letters

Basics · 6 min

Given a string s, return how many uppercase letters (A–Z) it contains.

In plain English

One pass; bump a counter for each character in the 'A'..'Z' range.

Examples

Example 1

Input: s = "Hello World"
Output: 2
Note: H and W.

Example 2

Input: s = "abc"
Output: 0
Note: None.

Example 3

Input: s = "ABC"
Output: 3
Note: All upper.

Constraints

0 ≤ s.length ≤ 10^4

Clarifying questions1First question: ASCII letters only?Explore questions

Q1.ASCII letters only?
Yes; Unicode uppercase would use isUpperCase.

Quick interview revision

Understand

30-second explanation

Build the interview-ready mental model before comparing implementations.

Scan and count characters where `ch >= 'A' && ch <= 'Z'`.

Interview contextWhat is testedWhere candidates slipHow to stand outExplore interview context

What's being tested

Whether you write a clean range-membership tally.

Where candidates slip

Range slip including lowercase or symbols.

How to stand out

Note it generalises to count-upper-lower with a second counter.

Choose

Compare the available approaches

Use constraints and trade-offs to select the route you can defend in an interview.

Learn & trace

Build each solution step by step

Move from the core idea to a visual trace, implementation, proof, and edge cases.

03.1

Approach 1 of 1 · Optimal

Range count

TimeO(n)SpaceO(1)

This is the standard, expected solution — lead with it.

Stuck? Progressive hintsReveal one at a time

1Hint 1Explore hint

What ASCII range holds the capital letters?

2Hint 2Explore hint

One counter, bump on a range match.

3Hint 3Explore hint

Ignore lowercase, digits, and symbols.

A · Core idea

Count characters in the uppercase ASCII range 'A'..'Z' with a single counter — the simplest range-membership tally.

Why a range check. ch >= 'A' && ch <= 'Z' is an O(1) test that includes only the 26 capital letters; lowercase letters, digits, spaces, and symbols all fail it and are ignored.

Why one pass, no early exit. A total count requires visiting every character. There's no shortcut without preprocessing.

It's the count-upper-lower problem reduced to just the uppercase bucket.

B · Visual dry run

Interactive dry run

Select a frame to connect the decision with its visual state.

s = "Hi World"

A-Z

Statecount=1

Final result2

C · Code with teaching comments

solution.javaJava

Range count — optimal

1
public int countUpper(String s) {
2
    int c = 0;
3
    for (char ch : s.toCharArray()) {
4
        if (ch >= 'A' && ch <= 'Z') c++;
5
    }
6
    return c;
7
}

D · Why it works and complexityO(n) timeO(1) spaceOpen the invariant and proofExplore the proof

ComplexityO(n) time, O(1) space — one range comparison per character.

E · Edge cases and pitfalls5s = "abc" — None → 0.Explore edge cases

Edge cases handled

s = "abc"None → 0.
s = "ABC"All upper → 3.
s = ""Empty → 0.

Pitfalls

Including lowercase letters by mistake.
Counting non-letters.

Practice

Try the solution yourself

Edit the optimal implementation, test another input, and observe the result.

Editable code previewpublic int countUpper(String s) { int c = 0; for (char ch : s.toCharArray()) { if (ch >= 'A' && ch <= 'Z') c++; }Explore the playground

Try it — Java· run a complete program; method snippets need a driver

Avoid mistakes

Recognize the interview traps

Understand why common attempts fail before reviewing the corrected implementation.

Including lowercase letters by mistake.
Counting non-letters.

Revise

Carry the pattern into the interview

Quick interview revision

Pattern to remember

ASCII-range count

Code pattern to memorize

c=0; for ch: if 'A'<=ch<='Z': c++

Rules to remember

1Uppercase: A-Z.
2One counter.
3Skip everything else.

Use this pattern when

Range-membership counting

Interview takeaway

Range-membership counting is one comparison per character.

Continue

Transfer the pattern to another problem

Use a direct variation or pattern sibling to prove the idea is reusable.

Transferable patternA core fresher pattern: ASCII-range count. Master it on small inputs and it transfers to harder problems.

Practice queueDirect variations and pattern siblings

Direct follow-ups

Count Upper and Lower
Add the lowercase counter.